oh yeah, flick is right, the package name is mozilla-firefox (just check portage), so it is:
emerge mozilla-firefox
not
emerge firefox
but you can search the package by typing
emerge --search firefox
or any package for that matter.
i apologize for the misinformation.

Ignorance is not shameful. Arrogantly ignoring and denying your ignorance is.
All men are capable of learning, except two: The Arrogant and The Shy.